1. Field of the Invention
This invention relates to a two-step process for brazing sleeves into heat-exchange tubes which avoids the creation of undesirable gaps in the braze joint.
2. Description of the Prior Art
Processes for brazing sleeves within the heat exchange tubes of nuclear steam generators are known in the prior art. In such processes, a sleeve which is circumscribed by a ring of brazing alloy at one or both ends is slid across a section of the tube whose walls are in need of repair due to corrosion or denting. The end of the tube having the ring of brazing alloy is then typically hydraulically expanded so that it snugly engages the inner walls of the tube. In order to complete a watertight seal between the sleeve and the interior walls of the tube, a brazing heat is applied across the expanded end of the sleeve. The brazing alloy melts and forms a braze joint between the sleeve and the interior walls of the tube. Such sleeving operations are often performed in the longitudinal sections of the heat exchange tubes which extend through the openings in the tubesheet and support plates of the nuclear steam generator due to the tendency of the walls of these tubes to corrode and dent in these regions.
While the aforementioned one-step brazing process frequently results in watertight braze joints, problems may arise when sludge deposits become tightly wedged in the annular space between the walls of these tubes and the bores or other openings in the tubesheets and support plates through which these tubes extend. Under such circumstances, the sludge deposits may bind the outer walls of the tube so tightly against the walls of the bores in the tubesheets and support plates that the tubes are prevented from expanding longitudinally when the brazing heat is applied to the tube and sleeve. Consequently, these tubes will expand radially instead of longitudinally in the area where the brazing heat is applied. Such a radial expansion in the area of the braze joint creates undesirable gaps in the joint which may seriously jeopardize the integrity of the watertight seal the braze joint is intended to produce. Of course the extent to which such undesirable gapping occurs varies widely between any group of sleeved tubes due to the varying amount of binding forces the sludge deposits surrounding these tubes apply when a brazing heat causes them to expand. However, in those tubes where the sludge deposits apply a substantial amount of resistance to longitudinal tube expansions, the amount of gapping which occurs will, at the very least, significantly jeopardize the ability of the braze joint to create the intended watertight seal.
Accordingly, there is a need for a brazing process which is capable of reliably providing a watertight braze joint between a sleeve and a tube in a heat exchanger without seal-jeopardizing gaps. Moreover, it would be desirable if this process were capable of uniformly producing watertight braze joints between the sleeves and tubes of any steam generator regardless of variations in the extent to which sludge deposits resisted longitudinal movements in the tube when a brazing heat was applied thereto. Finally, it would be desirable if this process could be easily implemented with existing brazing tools.